Nickel-titanium alloy with a non-alloyed dispersion and methods of making same

ABSTRACT

An article having a nickel-titanium alloy and a homogeneous dispersion of discrete particles. The discrete particles are substantially free of nickel and titanium. A method of making the article includes melting a substantially equiatomic composition of nickel and titanium to form an alloy and dispersing a discrete particle in the alloy to form an ingot. The melting and dispersing are performed at a temperature above the alloying temperature of the composition and below the melting temperature of the discrete particle. The ingot is hot worked to form a processed ingot. The processed ingot is cold worked and annealed to form the article.

FIELD OF THE INVENTION

The material and methods disclosed herein relate to a nickel-titaniumalloy, and a nickel-titanium alloy with a dispersion of discreteparticles substantially free of nickel and titanium.

BACKGROUND OF INVENTION

Nickel-titanium alloy, more specifically nitinol, is valued in a numberof industries because of its unique properties of superelasticity andshape memory. For example, nitinol is used in a number of components inminimally invasive surgery, including but not limited to catheters andstents.

While nitinol has its benefits, it also has its drawbacks. Particularly,nitinol is subject to brittle fracture. Once a fracture or crack isinitiated, it is subject to propagation under fatigue conditions. Thispropagation could have serious consequences, especially if the nitinolis being used in a medical device placed in a patient. Anotherdisadvantage of nitinol is that nitinol has low radiopacity, meaningthat the nitinol has limited visibility when viewed by x-ray basedmedical imaging systems. Low radiopacity and low resistance to fatiguecrack propagation limit the effectiveness of nitinol in the medicaldevice industry.

To overcome the disadvantages, development efforts have focused oncreating a ternary or higher order alloy from the base binary nitinol.Those efforts have achieved moderate improvement in nitinol properties,but the same drawbacks as with binary nitinol still exist.

SUMMARY OF INVENTION

One embodiment of the invention is an article having a nickel-titaniumalloy and a homogeneous dispersion of discrete particles in the article.The discrete particles are substantially free of nickel and titanium.

In another embodiment, the article is made by melting a substantiallyequiatomic composition of nickel and titanium to form an alloy anddispersing at least one type of discrete particles in the alloy to forman ingot. The melting and dispersing are performed at a temperatureabove the alloying temperature of the composition and below the meltingtemperature of the discrete particles. The ingot is hot worked to form aprocessed ingot. The processed ingot is cold worked and annealed to formthe article.

In a further embodiment, an article having a nickel-titanium alloy and ahomogeneous dispersion of discrete particles substantially free ofnickel and titanium is made by vacuum induction melting a substantiallyequiatomic composition of nickel and titanium to form a melted ingot.The melting is performed at or above an alloying temperature for thecomposition. During the vacuum induction melting, the discrete particlesare pour stream injected into the melted ingot at a temperature belowthe melting point of the discrete particles. The melted ingot is hotworked to form a processed ingot. The processed ingot is cold worked andannealed to form the article.

In yet another embodiment, an article having a nickel-titanium alloy anda homogeneous dispersion of discrete particles substantially free ofnickel and titanium is made by preparing an electrode having a hollowcenter. The electrode is made up of a substantially equiatomiccomposition of nickel and titanium. Inclusion particles are introducedinto the hollow center. The electrode and particles are vacuum arcmelted at a temperature above the alloying temperature of thecomposition and below the melting point of the particles to form amelted ingot. The melted ingot is hot worked to form a processed ingot.The processed ingot is cold worked and annealed to form the article.

BRIEF DESCRIPTION OF DRAWINGS

For the purpose of illustrating the invention there are shown in thedrawings various forms which are presently preferred; it beingunderstood, however, that this invention is not limited to the precisearrangements and instrumentalities particularly shown.

FIG. 1 shows an article according to one embodiment of the inventiondisclosed herein.

DETAILED DESCRIPTION OF INVENTION

With reference to the drawings, where like numerals identify likeelements, there is shown in FIG. 1 an article 10 in accordance with thematerials and methods disclosed herein. As illustrated, the article 10is a stent. While the article is shown as a stent, the article is not solimited. The article can be all or just a portion of any structurehaving the elements described herein. The article can be a productincluding, but not limited to, actuators, hydraulic line couplings,electrical connectors, fishing lures, eyeglass frames and golf clubs.Preferably, the article is a medical device including, but not limitedto, catheters, biopsy sectioning and retrieval equipment, vena cavafilters, and stents.

The article 10 comprises an alloy component 12. The alloy componentcomprises a nickel-titanium alloy. The nickel-titanium alloy can be abinary or higher alloy. Preferably, the nickel-titanium alloy is abinary “nitinol.” Nitinol includes a family of nickel-titanium alloyshaving a substantially equiatomic composition of nickel and titanium.The equiatomic composition results in an ordered crystalline structurewith the unique property of deformation with a high degree ofrecoverable (or pseudoelastic) strain, which allows the composition tobe returned to its original shape after deformation.

The unique properties of nitinol are referred to as superelasticity andshape memory. Superelasticity refers to the unusual ability of certainmetals/alloys to undergo large elastic deformation. When mechanicallyloaded, a superelastic nitinol article undergoes a recoverabledeformation up to very high strains (e.g., up to 8%). The load creates astress-induced martensitic transformation in the article. Uponunloading, a spontaneous reversal of the transformation occurs, causingthe article to return to its original shape. No change in temperature isneeded for the alloy to recover its initial shape.

In contrast to superelasticity, shape memory describes thecharacteristic that allows a plastically deformed article to be restoredto its original shape by heating it. The article is heated above theaustenite finish temperature of the article causing crystallinetransformation and returning the article to its original shape. Shapememory is important to many nitinol-based products. For example, in themedical device industry, the shape memory property is beneficial forreusable medical instruments. Medical personnel can shape the instrumentto fit the desired need (e.g., fit the patient's physiology). After use,the instrument can be heat sterilized, which results in the instrumentreturning to its original shape for future use.

Despite its beneficial characteristics, nitinol has low radiopacity andhigh susceptibility to fatigue crack propagation. Both the lowradiopacity and high susceptibility to fatigue crack propagation aredetrimental to the use of nitinol in products such as medical implants.

To increase radiopacity and decrease susceptibility for fatigue crackpropagation, the article 10 includes a homogeneous dispersion ofdiscrete particles 14. As used herein, the discrete particles are anymaterial other than nickel and titanium. Also as used herein, thediscrete particles do not form an alloy with the nickel and/or titaniumfrom the alloy component 12 of the article 10. The discrete particlesare preferably free of nickel and titanium. However, the discreteparticles can be “substantially free” of nickel and titanium.Substantially free means that some diffusion from the alloy componentoccurs, but that the diffusion is minimal. The discrete particles can beone or more elements including, but not limited to, iridium, platinum,gold, rhenium, tungsten, palladium, rhodium, tantalum, silver,ruthenium, and hafnium. The discrete particles can be one or more alloyscontaining one or more elements including, but not limited to, iridium,platinum, gold, rhenium, tungsten, palladium, rhodium, tantalum, silver,ruthenium, and hafnium.

As shown in FIG. 1, the discrete particles 14 form islands in the alloycomponent. The islands are substantially free of nickel and titanium anddo not form alloys with the nickel and titanium of the alloy component.The islands can be formed in any geometric or non-geometric shape.Preferably, the islands are spherical in shape. The islands can belocated anywhere in the article. Preferably, the islands are locatedsuch that the major axis of the discrete particles in the island arealigned perpendicular to the likely direction of fatigue crackpropagation in the article.

The materials described herein provide several benefits over binarynitinol, and over nitinol alloyed with other elements to form aconventional ternary or high order alloy. For example, the discreteparticles will have significantly lower impact upon the phasetransformational changes of the nitinol that result in superelasticand/or shape memory effects.

Another benefit of the material is increased resistance to crackpropagation. If a crack is formed in an article without particles, thecrack will continue unabated thereby damaging and/or destroying thearticle. However, if a crack front encounters a discrete particle,further propagation will be halted, thereby preserving the integrity ofthe article.

If the particle is chosen from the aforementioned list of elements oralloys, then there is an additional benefit of increasing theradiopacity of the resulting system over the radiopacity of the binarynitinol alloy alone. Increased radiopacity will allow physicians orother medical personnel to view medical devices containing the materialwith greater clarity using x-ray based imaging techniques.

Several methods are contemplated for making the article 10. The methodspreferably produce an article having a nickel-titanium alloy anddispersion of discrete particles in the article, wherein the discreteparticles are substantially free of nickel and titanium and do not forman alloy with nickel and/or titanium of the nickel-titanium alloy.

Generally, the methods include a step that includes melting asubstantially equiatomic composition of nickel and titanium to form analloy and dispersing at least one type of discrete particles in thealloy to form an ingot. The melting and dispersing are performed at atemperature above the alloying temperature of the composition and belowthe melting temperature of the at least one type of discrete particle.

When the Ti concentration is close to 50 weight percent, molten nitinolis highly reactive and must be melted in a vacuum. Vacuum inductionmelting (VIM) and vacuum consumable arc melting (VAR) are preferred waysto vacuum melt the materials.

VIM involves melting a metal composition under vacuum conditions byinducing alternating electrical eddy currents in the metal. Thecomposition is placed in an electrically conductive crucible, preferablygraphite or calcia or in a induction melter without a crucible. Thecomposition is placed in a vacuum chamber. The composition is heated byeddy currents causing the composition to melt.

The vacuum chamber can be a furnace having an air-tight, water-cooledsteel jacket that is capable of withstanding the vacuum required formelting. The inside of the furnace is typically lined with refractorymaterials.

VIM or vacuum skull melting (VSM) can be used in an embodiment of thepresently disclosed method. In that embodiment, a substantiallyequiatomic composition of nickel and titanium is vacuum induction meltedto form a melted ingot. The melting is performed at or above an alloyingtemperature for the composition. During the vacuum induction melting,particles are pour stream injected into the melted ingot. The injectionis performed at a temperature below the melting point of the particles.If more than one type of particle is injected, then the temperature isbelow the lowest melting point of all of the particles.

Alternatively, VAR can be used in place of VIM or VSM. In such anembodiment, an electrode is prepared from a substantially equiatomiccomposition of nickel and titanium. The electrode is prepared having ahollow center into which particles are introduced. The particles can beintroduced by injecting the particles into the hollow center, by packinga powder form of the particles into the hollow center, or any otherknown manner. After the particles are introduced, the electrode isvacuum arc melted to form a melted ingot. The vacuum arc melting isperformed at a temperature above the alloying temperature of thecomposition and below the melting point of the particles.

In a further alternative, VAR can be used in conjunction with VIM orVSM. In such an alternative, VIM or VSM melting is typically preformedfirst, followed by VAR melting. With that order, the VIM melting createsan ingot. The ingot is then used as a consumable electrode in the VARmelting. The VIM/VAR combination combines the benefits of VIM (e.g.,thorough mixing) with the benefits of VAR (e.g., high purity of theresultant alloy).

After the melted ingot is formed by any of the methods described herein,the ingot is typically refined by additional deformation processes inorder to optimize the beneficial properties, such as shape memory,superelasticity, or resistance to fracture. The additional deformationis typically done by first hot working the ingot to form a processedingot with a useful shape, while at the same time changing themicrostructure in the processed ingot into one that has optimizedbeneficial properties. Hot working can include, for example, pressforging, rotary forging, extrusion, swaging, bar rolling, rod rolling,or sheet rolling. Hot working is typically performed at temperatures inthe range of 70 to 85% of the alloying temperature of the nitinol.

To further optimize the beneficial properties, the processed ingot canundergo a series of cold working steps. The cold working steps providethe final shape, surface finish, refined microstructure, and mechanicalproperties of the article. Preferably, cold drawing or cold rolling areused to cold work the processed ingot. Typically a cold working step isfollowed immediately by an annealing step. The annealing step istypically performed at about 600° C. to about 850° C. Cold working andsubsequent annealing can be repeated multiple times, if necessary.

The result of each of the methods disclosed herein is an article havinga nickel-titanium alloy and a homogeneous dispersion of discreteparticles substantially free of nickel and titanium.

Optionally, as a finishing step, the article can be treated further byheat treatment. The heat treatment is typically done at about 450° C. toabout 550° C. The heat treatment is generally necessary when, after thecold working step, the article does not fully exhibit the desiredbeneficial properties.

It will be appreciated by those skilled in the art that the presentinvention may be practiced in various alternate forms andconfigurations. The previously detailed description of the disclosedmethods is presented for clarity of understanding only, and nounnecessary limitations should be implied therefrom.

1. An article comprising: a nickel-titanium alloy; and a homogeneousdispersion of discrete particles in the article, wherein the discreteparticles are substantially free of nickel and titanium.
 2. An articleaccording to claim 1, wherein the discrete particles are selected fromthe group consisting of iridium, platinum, gold, rhenium, tungsten,palladium, rhodium, tantalum, silver, ruthenium, and hafnium.
 3. Anarticle according to claim 1, wherein the discrete particles compriseone or more alloys substantially free of nickel and titanium.
 4. Anarticle according to claim 3, wherein the one or more alloys compriseone or more of iridium, platinum, gold, rhenium, tungsten, palladium,rhodium, tantalum, silver, ruthenium, and hafnium.
 5. An articleaccording to claim 1, wherein the discrete particles are alignedperpendicular to the likely direction of fatigue crack propagation. 6.An article according to claim 1, wherein the alloy comprises a binaryalloy.
 7. An article according to claim 1, wherein the alloy comprises aternary or higher alloy.
 8. An article according to claim 1, wherein thealloy is nitinol.
 9. An article according to claim 1, wherein thearticle is a medical device.
 10. An article according to claim 9,wherein the medical device is a stent.
 11. A method of making anarticle, the method comprising: melting a substantially equiatomiccomposition of nickel and titanium to form an alloy and dispersing atleast one type of discrete particle in the alloy to form an ingot,wherein the melting and dispersing are performed at a temperature abovethe alloying temperature of the composition and below the meltingtemperature of the discrete particles; hot working the ingot to form aprocessed ingot; and cold working and annealing the processed ingot toform the article.
 12. A method of making an article, the methodcomprising: vacuum melting a substantially equiatomic composition ofnickel and titanium to form a melted ingot, wherein the melting isperformed at or above an alloying temperature for the composition; pourstream injecting particles into the melted ingot during vacuum inductionmelting at a temperature below the melting point of the particles; hotworking the melted ingot to form a processed ingot; and cold working andannealing the processed ingot to form the article, the articlecomprising a nickel-titanium alloy and a homogeneous dispersion ofdiscrete particles substantially free of nickel and titanium.
 13. Amethod of making an article according to claim 12, wherein thesubstantially equiatomic composition has a nickel:titanium atomic ratioof about 50:50.
 14. A method according to claim 12 wherein the particlesare selected from the group consisting of iridium, platinum, gold,rhenium, tungsten, palladium, rhodium, tantalum, silver, ruthenium, andhafnium.
 15. A method according to claim 12, wherein the particlescomprise an alloy comprising one or more of iridium, platinum, gold,rhenium, tungsten, palladium, rhodium, tantalum, silver, ruthenium, andhafnium.
 16. A method according to claim 12, wherein the vacuum meltingcomprises vacuum induction melting.
 17. A method according to claim 12,wherein the vacuum melting comprises vacuum skull melting.
 18. A methodof making an article, the method comprising: preparing an electrodehaving a hollow center, the electrode comprising a substantiallyequiatomic composition of nickel and titanium; introducing particlesinto the hollow center; vacuum arc melting the electrode and theparticles at a temperature above the alloying temperature of thecomposition and below the melting point of the particles to form amelted ingot; hot working the melted ingot to form a processed ingot;and cold working and annealing the processed ingot to form the article,the article comprising a nickel-titanium alloy and a homogeneousdispersion of discrete particles substantially free of nickel andtitanium.
 19. A method according to claim 18, wherein the particles areselected from the group consisting of iridium, platinum, gold, rhenium,tungsten, palladium, rhodium, tantalum, silver, ruthenium, and hafnium.20. A method according to claim 18, wherein the particles comprise analloy comprising one or more of iridium, platinum, gold, rhenium,tungsten, palladium, rhodium, tantalum, silver, ruthenium, and hafnium.21. A method according to claim 18, wherein the particles are introducedby injecting the particle into the hollow center.
 22. A method accordingto claim 18, wherein the particles are introduced by packing a powderedparticle into the hollow center.